A large-area X-ray system may be suitable for various applications, including safety systems for detailed industrial inspections, quality control, analysis and measurement, and detailed aviation safety inspections, and medical applications such as Computed Tomography (CT).
However, in a typical large-area X-ray system, it is very difficult to achieve a uniform distribution and flux of X-rays over a large area. Accordingly, the large-area X-ray system includes a physically moving system, which increases the size of the X-ray system and greatly degrades its structural efficiency.
A current X-ray source typically uses a thermal electron emitting system using a filament, and thermal electron emission requires very high operation temperature (typically, about 1500° C.). The high operation temperature shortens the lifespan of the filament and leads to a very slow response time (since time is required to warm up the filament prior to emission), high energy consumption, and a large size. In particular, in medical applications, X-rays are continuously emitted for longer than necessary due to the slow response time of thermal electron emission, thus irradiating the human body more than necessary.
FIG. 1 is a schematic cross-sectional view of a CT system taken as an example of a conventional large-area X-ray system.
Here, an X-ray source 100 rotates around an object 120, as indicated by an arrow, because of its small area. A detecting device 110 moves with the X-ray source 100.
According to this CT design, a complex mechanical system included in a scanning system increases the size of the CT system. Since X-rays L are continuously emitted from the X-ray source 100 as described above, a target 120 is irradiated for a long time upon large-area imaging.
A conventional thermal electron emission X-ray system using a filament has a dipolar structure having a cathode and an anode (i.e., a diode structure). More specifically, when electrons are emitted from the cathode, a high voltage is applied to the anode to accelerate the electrons. Accordingly, it is difficult to focus and control the electrons. In addition, isotropic emission of thermal electrons from the filament is conducive to inefficient collection of the electrons at the anode.
To solve these problems, recently, nano emitters such as a Carbon Nano Tube (CNT) have been widely used. The nano emitters are conductive emitters having a sharp end and obeying a field emission principle whereby the emitter emits electrons in a vacuum state in response to an electric field. The nano emitters emit electrons straight in the direction of the electric field, with excellent performance and very high efficiency.
A typical field emission X-ray system using nano emitters has a triode structure including an anode, a cathode, and a gate for inducing electron emission. However, if electrons from the nano emitters leak to the gate, the gate is thermally deformed by leakage current, degrading electron emission reliability.
In particular, since the flux of electrons emitted from the Nano emitters is not uniform, the system is unsuitable for a large area. Accordingly, typical multiple X-ray tubes using Nano emitters cannot be arranged for a large area because the tubes output a different flux of X-rays.
Even though the nano emitters are formed on a single plate having a large area, the flux of emitted electrons is not uniform across the electron beam emission area. Accordingly, it is difficult to implement an X-ray system capable of uniformly emitting electron beams over a large area.
In addition, a conventional scheme of adjusting the flux of emitted electrons through gate adjustment has the drawback of requiring a high driving voltage, in addition to the above problem of uniformity.